MEMS mirror system for laser printing applications

ABSTRACT

A MEMS mirror for a laser printing application includes providing a CMOS substrate including a pair of electrodes, and providing a reflecting mirror moveable over the substrate and the electrodes. Voltages applied to the electrodes create an electrostatic force causing an end of the mirror to be attracted to the substrate. A precise position of the mirror can be detected and controlled by sensing a change in capacitance between the mirror ends and the underlying electrodes.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims benefit under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 60/886,740, filed Jan. 26, 2007,entitled “MEMS mirror for laser printing application,” the disclosure ofwhich is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

Embodiments in accordance with the present invention relate generally tolaser printing. In particular embodiments, the invention provides amethod and apparatus for fabricating and operating a mirror coupled to aCMOS substrate for use in laser printing. In a specific embodiment, atleast a portion of the mirror structure is fabricated from a singlecrystal layer joined to the CMOS substrate through a wafer-level layertransfer process. Other embodiments of the present invention have a muchbroader range of applicability.

Laser printing applications have become widespread. Despite improvementsin the area of laser printing, there is a continuing need in the art forimproved methods and systems related to laser printing.

SUMMARY OF THE INVENTION

According to the present invention, techniques related generally to thefield of laser printing are provided. More particularly, the presentinvention relates to methods and systems for fabricating and operating amirror coupled to a CMOS substrate. Other embodiments of the presentinvention have a much broader range of applicability.

According to an embodiment of the present invention, a laser printingapparatus is provided. The laser printing apparatus includes a lasersource and a mirror configured to receive an incident laser beam fromthe source and reflect the laser beam toward a photosensitive element.The mirror includes a CMOS substrate bearing a first electrode and asecond electrode and a reflecting surface supported above the CMOSsubstrate and rotatable about an axis in response to an electrostaticattraction force between an end of the reflecting surface and the firstelectrode.

According to another embodiment of the present invention, a printingmethod is provided. The printing method includes causing laser light tobe reflected to a photosensitive element to alter an electronic state ata location on the photosensitive element, thereby causing an inkmaterial to adhere thereto. The printing method also includestransferring the ink material from the photosensitive element toreceiving medium and altering a position of the reflecting surface bygenerating an electrostatic attraction between an end of the reflectingsurface and an electrode on an underlying CMOS substrate. The electronicstate of a different location is changed and ink transferred to adifferent region of the paper.

According to an alternative embodiment of the present invention, ascanning mirror for a laser printing apparatus is provided. The scanningmirror includes a frame and a pair of fixed electrode finger setscoupled to the frame. Each of the fixed electrode finger sets includes aplurality of fixed fingers. The scanning mirror also includes a set oftorsion hinges coupled to the frame, a mirror plate coupled to thetorsion hinges, and a pair of moveable electrode finger sets coupled tothe mirror plate. Each of the moveable electrode finger sets includes aplurality of moveable fingers. The plurality of moveable fingers areconfigured to interlace with the plurality of fixed fingers.

According to another alternative embodiment of the present invention, amethod of fabricating a scanning mirror for a laser printing apparatusis provided. The method includes providing an SOI substrate having afirst silicon layer, a silicon oxide layer abutting the first siliconlayer, and a second silicon layer abutting the silicon oxide layer. Themethod also includes forming a pair of moveable electrode finger setsfrom a first portion of the second silicon layer and forming a mirrorplate from a second portion of the second silicon layer. The methodfurther includes forming a pair of fixed electrode finger sets from afirst portion of the first silicon layer and forming a mirror rotationregion from a second portion of the first silicon layer. Moreover, themethod includes removing the silicon oxide layer to form a mirrorstructure and mounting the mirror structure on an electrode substrate.

Numerous benefits are achieved using the present invention overconventional techniques. Some embodiments provide methods and systemsthat include one or more scanning mirrors with high bandwidth orthroughput, allowing their rapid actuation to move the reflected laserquickly along a photosensitive element. Another advantage offered byembodiments of the present invention is a compact size or footprint, asthe mirror surface can readily be fabricated utilizing existingtechniques to have a surface area only slightly larger than the diameterof the spot of light received from the laser source. Still anotheradvantage offered by embodiments of the present invention is low cost,as the mirrors can be readily fabricated in high volumes utilizingestablished semiconductor fabrication techniques. Depending upon theembodiment, one or more of these benefits may exist. These and otherbenefits have been described throughout the present specification andmore particularly below. Various additional objects, features, andadvantages of the present invention can be more fully appreciated withreference to the detailed description and accompanying drawings thatfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram of an embodiment of a laserprinting apparatus utilizing an embodiment of the present invention;

FIG. 2 is a simplified plan view of an embodiment of a dual-axisscanning mirror in accordance with an embodiment of the presentinvention;

FIG. 3 is a simplified plan view of an embodiment of a single-axisscanning mirror in accordance with an embodiment of the presentinvention;

FIGS. 4A-F are simplified cross-sectional views showing the steps of anembodiment of a process flow in accordance with the present inventionfor fabricating a MEMS mirror structure;

FIG. 5A is a simplified schematic view of one embodiment of a system inaccordance with the present invention for sensing positioning of amirror;

FIG. 5B is a plot of the relationship between the mirror rotation angleand the bias voltage according to an embodiment of the presentinvention;

FIG. 6 is a simplified schematic view of an embodiment of a circuit inaccordance with an embodiment of the present invention for sensing andcontrolling mirror position;

FIG. 7 is a simplified perspective illustration of a MEMS mirror systemaccording to an embodiment of the present invention;

FIG. 8 is a simplified perspective illustration of a portion of the MEMSmirror system illustrated in FIG. 7;

FIGS. 9A to 9C is a simplified process flow for the fabrication of aMEMS mirror system according to an embodiment of the present invention;

FIG. 9D is a simplified illustration of a portion of a torsion hingeaccording to an embodiment of the present invention;

FIG. 9E is a simplified perspective illustration of a MEMS mirror systemfabricated according to the method illustrated in FIGS. 9A to 9D; and

FIG. 9F is a simplified top view illustration of a MEMS mirror systemfabricated according to the method illustrated in FIGS. 9A to 9D; and

FIG. 10 is a simplified flowchart illustrating a process flow forfabricating a MEMS mirror according to an embodiment of the presentinvention.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments of the present invention provide MEMS structures useful forlaser printing applications. Combining MEMS technology with CMOScircuitry, it is possible to fabricate a moveable mirror structureuseful in a wide variety of applications. The examples described hereinare provided merely for purposes of illustration and are not intended tolimit embodiments of the present invention.

FIG. 1 is a simplified schematic diagram of an embodiment of a laserprinting apparatus 100 utilizing an embodiment of the present invention.Specifically, laser printing apparatus comprises a laser scanning unitor source 102 in optical communication with a mirror 104. Light incidentto mirror 104 is reflected to a photosensitive element 106, here arotating photoreceptor drum. A charging electrode 108 (here a coronawire), is configured to impart charge to the surface of thephotosensitive element. The rotating photoreceptor drum is in closeproximity to a toner coated roller 110.

Application of the laser beam reflected by the mirror 104, to a surfaceof the photosensitive element 106, results in a localized change in theelectrical state of the photosensitive element. This changed electricalstate causes toner to become attached to the local area, which is thenplaced into contact with the underlying moving paper 120. This contactresults in the toner being printed on the paper only in the local area.

Conventionally, the mirror element is created by machining or otherexpensive and time-consuming processes. As shown in FIG. 1, however,embodiments in accordance with the present invention propose tofabricate the mirror element 104 as a microelectromechanical structure(MEMS). Specifically, FIG. 1 shows MEMS mirror 104 comprising reflectivesurface 104 a connected with and rotatable about surrounding frames 105utilizing hinge pairs 107 and 109.

FIG. 2 is a simplified top-view of a MEMS scanning mirror according tothe embodiment of the present invention shown in FIG. 1. Mirror surface104 a in FIG. 2 is configured to be movable about two different axes. Inparticular, a first set of hinges 202 allow surface 104 a to rotateabout the X-axis, while second set of hinges 109 allow surface 104 a torotate about the Y-axis. Again, the various hinges and the moveableplate may all be fabricated from the continuous piece of materialprovided by the silicon layer according to embodiments of the presentinvention.

The shape of the moveable plate is illustrated as a circle in FIG. 2merely by way of example. In other embodiments, other shapes areutilized as appropriate to the particular mirror application. One ofordinary skill in the art would recognize many variations,modifications, and alternatives.

For example, FIG. 3 is a simplified top-view of a MEMS scanning mirror300 according to an embodiment of the present invention. Specifically asingle hinge pair 302 allows reflecting surface to rotate about theY-axis. The hinges and the moveable plate may all be fabricated from thecontinuous piece of material provided by the silicon layer according toembodiments of the present invention.

FIGS. 4A-F illustrates a simplified process flow used to fabricate ascanning mirror according to an embodiment of the present invention. Theprocess illustrated in FIGS. 4A-F is merely an example of a process flowand is not intended to limit the scope of embodiments of the presentinvention.

In FIG. 4A, a CMOS substrate 400 is provided that includes a number ofelectrodes 402 on a surface of the CMOS substrate. The electrodes areelectrically connected to other circuitry (not shown) in the CMOSsubstrate. Other components of the CMOS substrate are not illustratedfor purposes of clarity. In an embodiment, the CMOS substrate is a fullyprocessed CMOS substrate. Additional details of the fabricationprocesses for the device substrate are provided in co-pending andcommonly owned U.S. Pat. No. 7,022,245, filed Jan. 13, 2004, which ishereby incorporated by reference for all purposes. One of ordinary skillin the art would recognize many variations, modifications, andalternatives.

Although not illustrated in FIG. 4A, dielectric layer 404 is generallydeposited on the CMOS substrate 400 and then planarized. The dielectriclayer 404 may be formed from silicon oxide, silicon nitride, siliconoxynitride, combinations thereof, and the like. The upper surface of thedielectric layer is planarized using, for example, a CMP process, toform a bonding surface characterized by a predetermined surfaceroughness. In an embodiment, the surface roughness is less than 5 Å RMS.

Electrodes 402 are typically formed by the deposition and/or patterningof a metal layer. After formation of the electrodes, dielectric layer404, with a thickness of t₁, is deposited on substrate 400. Layer 404 isa silicon dioxide (SiO₂) layer in a specific embodiment of the presentinvention, but as described more above, this is not required by thepresent invention. Other suitable materials may be used within the scopeof the present invention. For example, layer 404 is formed by depositionof silicon nitride (Si₃N₄) or silicon oxynitride (SiON) layers inalternative embodiments. Moreover, polysilicon material, includingamorphous polysilicon, is deposited to form layer 404 in yet anotheralternative embodiment according to the present invention.

The deposited layer 404 has a predetermined thickness t₁ as initiallydeposited. In a specific embodiment, the thickness t₁ is 2.6 μm. Inother embodiments, the thickness ranges from about 1.0 μm to about 3.0μm. Of course, the thickness will depend on the particular applications.As illustrated in FIG. 4A, the upper surface 405 of the deposited layer404 is uniform across the substrate 400 in FIG. 4A, resulting in aplanar surface. However, a planar surface after deposition is notrequired by the present invention. In a particular deposition process,the patterned nature of the electrodes 402 results in the thickness oflayer 404 varying as a function of lateral position, producing an uppersurface 405 that is not entirely flat.

To planarize the upper surface 405 of the deposited layer 404, anoptional CMP step is performed in an embodiment of the presentinvention. The results produced by the CMP process are illustrated bythe polished surface in FIG. 4A. Material above the surface 405 isremoved during the CMP process, resulting in a highly polished andplanarized layer. In a particular embodiment, the root-mean-square (RMS)roughness of the planarized surface 405 is less than or equal to about 5Å. As will be described below, the extremely smooth surface producedduring the CMP process facilitates bonding of the composite substrate tothe device substrate. In embodiments according to the present invention,the height of the polished layer 404 is about 1.9 μm. Alternatively, theheight ranges from about 0.5 μm to about 2.5 μm in other embodiments. Ofcourse, the height will depend upon the particular applications.

In FIG. 4B, the dielectric layer 404 previously formed, is selectivelyetched to create spacer structures 406 positioned to support a moveablereflecting surface over the substrate and the electrodes. As illustratedin the figure, portions of the dielectric layer 404 have been removedduring the etch process, resulting in the formation of spacer structures406.

Embodiments of the present invention in which the spacer structures arefabricated from silicon oxide, silicon nitride, or silicon oxynitride,or combinations thereof, provide benefits based on the electrical andthermal properties of the dielectric material. For example, thesematerials, among others, provide a high degree of electrical insulation,electrically isolating the device substrate from layers supported by thespacer structures. Moreover, the thermal properties of the material usedto deposit layer 404, such as thermal insulation, are provided by someembodiments. Other suitable spacer structures materials, such aspolysilicon material, including amorphous polysilicon are characterizedby electrical and thermal properties that provide benefits inalternative embodiments.

As illustrated in FIG. 4B, an isotropic etch has been used to define thespacer structures 406. The etch profile defines vertical walls for thespacer structures with a predetermined thickness. In an embodiment, thelateral thickness of the spacer structures is 0.5 μm. In otherembodiments, the thickness of the spacer structures varies from about0.25 μm to about 1 μm. An etch process that terminates at the uppersurface of the electrodes 402 is used in an alternative process thatresults in simultaneous exposure of the electrodes and passivation ofthe surface of the substrate 400. In yet another embodiment, the etchingprocess is terminated prior to exposure of the electrode layer, enablingthe spacer structures to not only provide mechanical support in the formof the spacer structures, but additional passivation benefits to theelectrodes on substrate 400.

As discussed above, in some embodiments of the present invention, theprocesses used to deposit, pattern, and etch the layer or layers fromwhich the spacer structures are fabricated are performed at lowtemperatures. For example, these processing steps may be performed witha view to the structures present on the device substrate prior to theformation of the spacer structures, such as CMOS circuitry. Since someCMOS circuitry may be adversely impacted by performing high temperaturedeposition processes, which may damage metals coupling CMOS transistorsor result in diffusion of junctions associated with the CMOS circuitry,low temperature deposition processes are utilized according to someembodiments of the present invention. Moreover, in a particularembodiment of the present invention, low temperature deposition,patterning, and etching processes, such as processes performed attemperatures of less than 500° C., are used to form the layer or layersfrom which the spacer structures are fabricated. In another specificembodiment, deposition, patterning, and etching processes performed atless than 400° C., are used to form the layer from which the spacerstructures are fabricated. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives within thescope of low temperature processes.

As shown in FIG. 4C, a silicon-on-insulator (SOI) wafer 408 is broughtinto physical proximity to the CMOS substrate. In FIG. 4D, the SOI wafer408 is brought into contact with and bonded to spacer structures 406.Bonding can occur using a variety of techniques. In a specificembodiment, the bonding occurs using a room temperature covalent bondingprocess. Each of the faces is cleaned and activated, e.g., by plasmaactivation or by wet processing. The activated surfaces are brought incontact with each other to cause a sticking action. In some bondingprocesses, mechanical force is provided on each substrate structure topress the faces together. In embodiments in which an SOI substrate isused with a silicon layer and the spacer structures are silicon oxide,silicon bearing bonds are created between the two faces. In alternativeembodiments, an oxide layer is formed on the silicon surface of the SOIsubstrate prior to bonding to provide an oxide-oxide bond interface. Theupper surface of the layer from which spacer structures are formed ispolished by a CMP process in one embodiment while the bonding surface ofthe SOI substrate is polished as well, providing an extremely smoothsurface that is conducive to covalent bonding processes. Of course, oneof ordinary skill in the art would recognize many other variations,modifications, and alternatives.

Because the standoff regions and the electrodes are formed on substrate400, the alignment tolerances for the wafer bonding process are greatlyrelaxed in comparison to some conventional techniques. For example, insome embodiments of the present invention, the tolerance requirement foraligning the two substrates prior to joining is less than 1 cm.Tolerance requirements on the order of millimeters are thereforeavailable through embodiments of the present invention, in contrast totolerance requirements on the order of microns for some conventionalstructures.

In FIG. 4E, the bonded SOI substrate is subjected to a sequence ofprocesses to remove material to leave silicon reflecting surface 408 csupported by spacer structures 406. In particular, silicon 408 a on thebonded SOI substrate is removed quickly utilizing a grinding process,leaving a roughened silicon surface overlying the oxide layer 408 b.Next, the roughened silicon surface is removed selective to the oxidelayer 408 b by a chemical etchback process. The buried oxide layerserves as an etch stop in one embodiment of the present invention, inwhich the SOI substrate is thinned by an etch process.

The oxide layer 408 b is then removed utilizing a different chemicaletching process selective to oxide over silicon, yielding reflectingsilicon surface 408 c. Plasma ashing is used in some embodiments toremove the buried oxide layer and expose reflecting silicon surface 408c. Other material removal processes (e.g., CMP processes) are utilizedto expose the oxide layer in other embodiments. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives.

As shown in FIG. 4F, reflecting silicon surface 408 c is thenselectively etched to define gaps 409 separating individual mirrors 410.These mirrors in accordance with embodiments of the present inventionhave a width slightly greater than a diameter of the laser beam incidentthereto and reflecting therefrom. For example, individual etched mirrorsmay exhibit a width of between about 100 μm to about 500 μm. In otherembodiments, the width of the mirrors is about 4 mm by about 4 mm. Ofcourse, the particular dimensions will depend on the particularapplication.

As also shown in FIG. 4F, the reflecting silicon surface 408 c is alsoselectively etched to form gaps 411 within individual mirrors 410. Gaps411 define hinge portions 412 about which the individual mirrors canrotate. The orientation of the gaps can dictate the rotationalcapabilities of the reflecting surface, for example as shown above inFIGS. 2 and 3.

Embodiments of processes for fabricating a scanning mirror according tothe present invention are disclosed in U.S. Pat. Nos. 7,092,140,7,022,245, and 7,118,234, each of which is herein incorporated byreference.

As indicated in FIG. 1, it is important to sense the exact position ofthe scanning mirror in order to identify the precise location at whichprinting will occur. Accordingly, FIG. 5A is a simplified schematic viewof one embodiment of a system in accordance with the present inventionfor sensing positioning of the scanning mirror. The rotation angle of anvoltage-actuated mirror has a nonlinear dependence on the drive voltage.An example of the relationship between the mirror rotation angle and thebias voltage is illustrated in FIG. 5B. Additional discussion related tothe rotation angle is provided in U.S. Pat. No. 4,317,611 to Petersen).Typically, for a given design, within the first 30% of the maximumrotation angle, the mirror can be accurately positioned at a desiredangle under closed loop operation. However, beyond the exemplary 30%maximum rotation angle, it is difficult to maintain stable feedback andthe mirror could enter a “point of no return” and “snap” to the maximumangle of rotation due to the nonlinear response of the mirror to thedrive voltage.

In particular, FIG. 5A shows scanning mirror 500 supported over CMOSsubstrate 502 and rotatable about X-axis extending into and out of thepage. A constant bias voltage (V_(bias)) is applied to the mirror 500,together with a varying voltage component (V_(c) cos(ω_(t))) from avariable voltage generator 504.

CMOS substrate 502 bears electrode pair 506 a and 506 b. Theseelectrodes, together with the ends 500 a and 500 b of reflecting surface500 and the gaps L and L′ between the electrodes 504 a and 504 b,respectively and the reflecting surface, define a pair of electrodestructures 508 a and 508 b. These electrode structures exhibitcapacitances of C₁ and C₂, respectively.

During operation of the scanning mirror, the reflecting surface isactuated so that one end tilts closer to the substrate and the other endtilts away from the substrate. This allows incident laser light 510 tobe reflected toward the photosensitive element in the manner shown.

A change in position of the end of the reflecting surface relative to anunderlying substrate, will change the distance of the gap between theelectrode and the reflecting surface, thereby altering the capacitance.In accordance with an embodiment of the present invention, this changedcapacitance can be sensed to detect the precise position of the mirror.

In certain embodiments, the changed capacitance can be detected on thesame electrodes to which voltage is provided to actuate the position ofthe reflecting surface. In accordance with other embodiments, separateelectrodes may be used to detect and correct for the actual position ofthe mirror.

For example, FIG. 6 is a simplified schematic view of an embodiment of acircuit in accordance with an embodiment of the present invention fordetecting and controlling mirror position. Specifically, voltage supply504 provides variable voltage (V_(bias)+V_(c) cos(ω_(t))) to the ends ofthe movable reflecting surface.

Drive amplifiers 602 a and 602 b provide voltage to the electrodes 506 aand 506 b, respectively. A difference in voltage between the electrodes506 a and 506 b and ends 500 a and 500 b, respectively, of thereflecting surface 500 gives rise to a first electrode-mirrorcapacitance (cap. 1) and a second electrode-mirror capacitance (cap. 2).Voltages on the electrodes 506 a and 506 b are fed as inputs to sensingamplifier 604, which is configured to output a voltage signalcorresponding to the difference between them. Because any voltagedifference at the electrodes is attributable to a change in the gapbetween the mirror end and the electrodes, and hence an angle (θ) ofinclination of the mirror, the output of sensing amplifier x is afunction of θ: (V₁(θ)).

This output of the sensing amplifier is fed to node 606 in electricalcommunication with a reference voltage (Vr(θr) and with an input node ofa controller 608. The reference voltage is calibrated based upon knownvalues expected from a particular angle of inclination of the mirror.Based upon the voltage received at the input node 608 a of thecontroller 608, the controller outputs a pair of voltages to the driveamplifiers 602 a and 602 b respectively, to bias the reflecting surfaceto the angle that is expected based upon the reference voltage. In thismanner, the circuit shown in FIG. 6 provides a closed loop systemutilizing capacitance to detect any deviation of the mirror positionfrom its expected value, and correct that deviation.

FIG. 7 is a simplified perspective illustration of a MEMS mirror systemaccording to an embodiment of the present invention. FIG. 8 is asimplified perspective illustration of a portion of the MEMS mirrorsystem illustrated in FIG. 7. Referring to FIG. 7, a number of moveablefingers 710 and a number of fixed fingers 720 form an electrostatic combactuator that is configured to rotate the mirror plate 725 about an axisdefined by torsion hinges 730. Although only a single torsion hinge isillustrated in FIG. 7, an additional torsion hinge opposing theillustrated torsion hinge is provided on the back side of the mirrorplate. The present invention utilizes vertical or “out of plane”electrostatic comb actuators, which differ in some respects from lateralcomb actuators commonly found in various sensors such as accelerometers.The embodiment illustrated in FIG. 7 provides benefits including lowactivation voltage in comparison to some MEMS designs due to the smallspacing between the moveable fingers and the fixed fingers.Additionally, the large surface area provided by the moveable and fixedfingers provides for increase electrostatic forces for a givenseparation distance. Thus, embodiments of the present invention providefor higher rotational bandwidths for a given electrode voltage incomparison with some MEMS designs. Moreover, because of the largeelectrostatic forces achieved by embodiments due, in part, to the closeelectrode spacing and large electrode surface area, stiffer torsionhinges are utilized by the designs described herein, further increasingthe rotational bandwidth over some MEMS designs.

In a specific embodiment, the number of finger pairs is about 100 pairs.The particular number of the geometry illustrated in FIG. 7 enables themirror plate 725 to rotate clockwise and counter-clockwise in responseto actuation by the comb actuator. As illustrated in FIG. 7, the mirrorplate is tilted clockwise at an angle of about 15 degrees. According toembodiments of the present invention, the mirror plate is able to rotateabout ±20 degrees in response to electrostatic actuation.

In the embodiment illustrated in FIGS. 7 and 8, the height 740 of thefixed fingers is about 700 μm, the height 742 of the moveable fingers isabout 50 μm, the thickness 744 of the mirror plate is about 10 μm, thewidth of the fixed fingers and the moveable fingers is about 5 μm, andthe length 748 of the fixed fingers and moveable fingers is about 200μm. Of course, in other embodiments, to provide desired rotation anglesand electrostatic forces, the various dimensions could be varieddepending on the particular application. The mirror's dynamicdeformation is a function of mirror size, mirror thickness, scanningfrequency, rotational angle, and the like. Adding a meshed pattern inthe back of the mirror can help reduce the peak-to-peak dynamicdeformation to a level smaller than 1/10 of the wavelength, therebypreventing diffraction from limiting the optical performance of thescanning mirror. It will be appreciated that by utilizing one or moreadditional masking steps, meshed or other patterns can be implemented inthe various structures described herein, for example, the structuresillustrated in FIGS. 2-5 and 7. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives. The mirrorillustrated in FIG. 7 is square in shape, with a top surface of 4 mm by4 mm, but this is not required by embodiments of the present invention.Depending on the printing application, the surface area of the mirrorwill be selected as a function of the light beams used in the printingprocess, the scan rate, the optical system, and the like.

Referring to FIG. 8, the moveable fingers 710 are attached to the mirrorplate 725 with the top of the mirror plate flush with the tops of themoveable fingers. In another embodiment, which is described more fullyin relation to the fabrication processes described below, the bottom ofthe mirror plate is flush with the bottoms of the moveable fingers.Depending on the process flow utilized to fabricate the scanning mirrorand comb actuator, either configuration can be utilized. Upon actuation,the moveable fingers move vertically between the fixed fingers, therebyrotating the mirror plate about the torsion hinge 730. The presentinventors have determined that in one configuration, if the electrodesare biased at a voltage of about 200 V, the mirror plate can rotate to±19.4 degrees at a rate of 4.5 kHz. Depending on the degree ofrotational damping provided by the mirror plate, torsion hinge, and/orthe environment of the mirror package, the rotation rate can vary,providing other rotation rates suitable to the particular application.

The scanning mirror illustrated in FIG. 7 is fabricated and then mountedon an electrode substrate (not shown), which may be a CMOS substrate orother suitable substrate having electrode contact locations. The fixedfinger electrodes are provided with electrical contact to one or moreelectrodes contacts present on the substrate and thereby electricalcommunication between drive electronics and the fixed finger electrodesis provided. Additionally, one or more separate electrode contacts areprovided in order to provide electrical communication to the moveablefinger electrodes. In an embodiment, the electrical communication withthe moveable finger electrodes is provided through the frame, thetorsion hinges, and the mirror plate. It should be noted that thecapacitive position sensors described above can be utilized incombination with the scanning mirror with vertical comb actuatorsdescribed herein.

FIGS. 9A to 9D is a simplified process flow for the fabrication of aMEMS mirror system according to an embodiment of the present invention.The fabrication process utilizes a silicon-on-insulator (SOI) substratehaving a first single crystal silicon layer approximately 700 μm thick,an oxide layer, approximately 1-2 μm thick, and a second single crystalsilicon layer approximately 50 μm thick. As described more fully below,the moveable fingers 710 are fabricated in the 50 μm thick second layerwhile the fixed fingers 720 are fabricated in the 700 μm thick firstlayer. Although single crystal silicon layers are utilized in theembodiment described herein, this is not required by embodiments of thepresent invention since layers containing other materials such aspolysilicon, silicon nitride, and the like are included within the scopeof the present invention.

In the process illustrated in FIG. 9A, the moveable fingers and thetorsion hinge are masked and etched in the second single crystal siliconlayer. The masking step is not illustrated for purposes of clarity. Thedimensions of the moveable fingers and the torsion spring hinge areselected in accordance with design considerations including electrodearea, hinge stiffness, and the like.

In the embodiment illustrated in FIG. 9A, the etching or otherprocess(es) utilized to remove silicon material between the moveablefingers and along the edge of the mirror plate is a reactive ion etch(REI) process that is timed to end at the oxide layer 910. In otherembodiments, combination physical/chemical etch processes are utilizedthat remove the majority of the silicon material using an RIE processand then expose the oxide layer utilizing a preferential chemical etch.The mask layer is removed after completion of the etching processes.

In order to define the thickness of the mirror plate, the first singlecrystal silicon layer is masked and the central portions o the firstsingle crystal silicon layer is removed using an etching or othersuitable process. In the illustrated embodiment, the layer is etchedusing a timed etch that removes approximately 40 μm of material, leavingabout 10 μm of silicon material remaining. In other embodiments, theparticular time and/or other etch process parameters are selected toprovide the desired thickness of the mirror plate. Thus, although athickness of 10 μm is utilized in one embodiment, this is not requiredby the present invention. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

FIG. 9C is a simplified illustration of the fabrication of the fixedfingers according to an embodiment of the present invention. The SOIsubstrate has been flipped over so that the moveable fingers are facingdown. Then, the first single crystal silicon layer is masked and etchedor otherwise processed to remove the material between the fixed fingersas well over the already defined mirror plate. Thus, the illustratedetch step removes approximately 700 μm of material, exposing the oxidelayer 910. As discussed in relation to FIG. 9A, a combinationphysical/chemical etch process may be utilized to remove the majority ofthe silicon material of the first single crystal silicon layer using anRIE process and then expose the oxide layer utilizing a preferentialchemical etch. One of ordinary skill in the art would recognize manyvariations, modifications, and alternatives. The mask layer is removedafter completion of the etching process. The oxide layer of the SOIsubstrate is removed to release the mirror structure and enable themoveable fingers to slide between the fixed fingers in response toelectrostatic actuation. The torsion spring hinges, which are attachedto a frame structure, illustrated in FIGS. 9E and 9F, are also releasedwhen the oxide layer is removed.

FIG. 9D is a simplified illustration of a portion of a torsion hingeaccording to an embodiment of the present invention. As illustrated inFIG. 9D, the hinge 950 extends away from the mirror plate 960 to attachto a frame structure (not illustrated). The structure illustrated inFIG. 9D would be present after the removal of the material in thecentral portion of the second single crystal silicon layer, therebydefining the thickness of the mirror plate. The particular geometry ofthe torsion spring hinge, 50 μm tall, 15 μm wide (reference number 952),and 10 μm long (reference number 954) are defined during the masking andetching processes described previously.

FIG. 9E is a simplified perspective illustration of a MEMS mirror systemfabricated according to the method illustrated in FIGS. 9A to 9D. Framestructure 970 is attached to the torsion spring hinges 950 and mountedon an electrode substrate (not shown) that includes electrical contactpads in electrical communication with the fixed fingers as well as themoveable fingers and the mirror plate. The frame structure provides formechanical support for the fixed fingers as well as the torsion springhinges, which, in turn, provide mechanical support to the mirror plateand moveable fingers.

FIG. 9F is a simplified top view illustration of a MEMS mirror systemfabricated according to the method illustrated in FIGS. 9A to 9D. InFIG. 9F, the mechanical connection between the frame structure and thetorsion spring hinges is illustrated. The right side of the mirror plate990 moves into the page upon activation in a first rotation direction(e.g., clockwise rotation). The left side of the mirror plate moves intothe page upon activation in a second rotation direction (e.g.,counter-clockwise rotation) opposed to the first rotation direction.

FIG. 10 is a simplified flowchart illustrating a process flow 1000 forfabricating a MEMS mirror according to an embodiment of the presentinvention. Moveable fingers and hinges are formed in the substrate(e.g., an SOI substrate) (1010). In a particular embodiment, themoveable fingers and torsion spring hinges are etched from a singlecrystal silicon layer of the SOI substrate after a masking step isperformed. This etching process exposes the insulator layer of the SOIsubstrate, for example, the silicon oxide layer. A mirror plate isformed (1012) by removing additional material from the layer from whichthe moveable fingers and the hinges were formed. As will be evident toone of skill in the art, additional masking and mask removal steps willbe utilized as appropriate.

The SOI substrate is flipped over to provide access to the other siliconlayer of the SOI substrate. A masking and removal (typically etching)process is utilized to form the fixed fingers from the other siliconlayer (1016). In embodiments of the present invention, the fixed fingersare significantly thicker than the moveable fingers. The mirror rotationregion is also formed (1016) by removing a portion of the other siliconlayer. As illustrated in FIG. 7, the central portion of the 700 μm thicklayer is removed to enable the mirror to rotate freely in both clockwiseand counter-clockwise directions.

The mirror structure is released (1018) using a chemical etching processthat removes the silicon oxide insulating layer of the SOI substrate.The removal of the oxide layer between the moveable and fixed fingersenables the moveable fingers to move both vertically and laterally sincethey are interlaced with the fixed fingers. After fabrication of themirror structure, the structure is mounted to an electrode substrate(e.g., a CMOS substrate) that includes electrodes and contact padsconfigured to provide electrical signals to the MEMS mirror structure.In an embodiment, the oxide in the area of the moveable fingers andhinges is selectively removed to maintain the oxide under the hinges andthereby provide a mechanical connection to the bottom substrate. Thisincludes, in one example, definition of a large open area and control ofthe wet etch time.

It should be appreciated that the specific steps illustrated in FIG. 10provide a particular method of fabricating a MEMS mirror according to anembodiment of the present invention. Other sequences of steps may alsobe performed according to alternative embodiments. For example,alternative embodiments of the present invention may perform the stepsoutlined above in a different order. Moreover, the individual stepsillustrated in FIG. 10 may include multiple sub-steps that may beperformed in various sequences as appropriate to the individual step.Furthermore, additional steps may be added or removed depending on theparticular applications. One of ordinary skill in the art wouldrecognize many variations, modifications, and alternatives.

In an alternative embodiment, the mirror plate is aligned with the topof the moveable fingers. Referring to FIG. 8, the embodiment illustratedin relation to the process flow in FIGS. 9A-9D and FIG. 10 has themirror plate aligned with the bottom of the moveable fingers. In orderto fabricate a mirror structure with the mirror plate aligned with thetop of the moveable fingers, the moveable fingers and the torsion springhinges are defined using an etching or other material removal process.The SOI substrate is then flipped to provide access to the thickersilicon layer. The stationary or fixed fingers are then etched alongwith the cavity in the central portion of the thicker silicon layer,thereby providing for a rotation space for the mirror plate. Theinsulator (e.g., silicon oxide) layer of the SOI substrate is removed,for example, through the use of a chemical or other etching process.

The thickness of the mirror plate is defined by etching from the side ofthe mirror structure including the fixed fingers. Utilizing a timedetch, a portion of the thinner silicon layer is removed, forming amirror plate with the top aligned with the top of the moveable fingers.One of ordinary skill in the art would recognize many variations,modifications, and alternatives.

While the present invention has been described with respect toparticular embodiments and specific examples thereof, it should beunderstood that other embodiments may fall within the spirit and scopeof the invention. The scope of the invention should, therefore, bedetermined with reference to the appended claims along with their fullscope of equivalents.

1. A scanning mirror for a laser printing apparatus, the scanning mirrorcomprising: a frame; a pair of fixed electrode finger sets coupled tothe frame, each of the fixed electrode finger sets comprising aplurality of fixed fingers; a set of torsion hinges coupled to theframe; a mirror plate coupled to the torsion hinges; and a pair ofmoveable electrode finger sets coupled to the mirror plate, each of themoveable electrode finger sets comprising a plurality of moveablefingers, wherein the plurality of moveable fingers are configured tointerlace with the plurality of fixed fingers, wherein the movablefingers are electrically coupled to a CMOS substrate via the frame, thetorsion hinges, and the mirror plate.
 2. The scanning mirror of claim 1wherein: the plurality of fixed fingers are characterized by a first setof opposing faces having a surface area greater than other faces of theplurality of fixed fingers; the plurality of moveable figurescharacterized by a second set of opposing faces having a surface areagreater than other faces of the plurality of moveable fingers; and thefirst set of opposing faces and the second set of opposing faces areparallel when interlaced.
 3. The scanning mirror of claim 1 wherein theframe and the pair of fixed electrode fingers comprise a single crystalsilicon material.
 4. The scanning mirror of claim 1 wherein the pair ofmoveable electrode fingers comprise a single crystal silicon material.5. The scanning mirror of claim 1 wherein a surface of the mirror plateis coplanar with a surface of the pair of moveable electrode fingersets.
 6. The scanning mirror of claim 1 wherein the set of torsionhinges is coplanar with the pair of moveable electrode finger sets. 7.The scanning mirror of claim 1 wherein the mirror plate has a rotationrange of between −20 degrees and +20 degrees.
 8. The scanning mirror ofclaim 1 wherein a surface area of the mirror plate varies as a functionof one of a light beam incident on the mirror plate, a scan rate, oroptics used in the laser printing apparatus.
 9. The scanning mirror ofclaim 1 wherein a bottom surface of the mirror plate is co-planar with abottom surface of the movable finger sets.
 10. The scanning mirror ofclaim 1 further comprising a support structure formed on a bottomsurface of the mirror plate, the support structure being configured toreduce deformation of the mirror plate.
 11. The scanning mirror of claim1 wherein an electrostatic force between the fixed fingers and themovable fingers is a function of a first surface area of the fixedfingers, a second surface area of the movable fingers, and a separationdistance between each of the fixed fingers and the movable fingers. 12.The scanning mirror of claim 1 wherein a thickness of the mirror plateis about 10 microns.
 13. The scanning mirror of claim 1 wherein a heightof each of the fixed fingers is about 700 microns.
 14. The scanningmirror of claim 1 wherein a height of each of the movable fingers isabout 50 microns.
 15. The scanning mirror of claim 1 wherein each of thefixed fingers and the movable fingers have a width of about 5 microns.16. The scanning mirror of claim 1 wherein each of the fixed fingers andthe movable fingers have a length of about 200 microns.
 17. The scanningmirror of claim 1 further comprising an oxide structure disposed betweenthe torsion hinges and the CMOS substrate, wherein the electricalconnection from the movable fingers to the CMOS substrate passes throughthe oxide structure.